Transmission performance of long-haul optical communication systems such as transoceanic systems is limited by a number of phenomena, including noise, dispersion, and nonlinearities in the refractive index of the fiber. Such systems require dispersion management techniques to mitigate the effects of the fiber's nonlinear index of refraction. Dispersion management for a long transmission line is a technique in which the zero dispersion wavelength of the constituent fibers is arranged to be appropriately far from the system's operating wavelengths while maintaining an appropriately small net dispersion for the whole transmission line. This arrangement is employed because at the zero dispersion wavelength, the signal and the amplified spontaneous emission noise generated by the optical amplifiers travel at similar velocities and thus have the opportunity to mix over long interaction lengths, via the nonlinear refractive index, and generate power at unwanted wavelengths. These interactions can be reduced if the group velocities of the signal and the noise are different so that there is a reduction in the distance over which the interactions occur. Accordingly, fibers having nonzero dispersion are often used to overcome the problems caused by fiber nonlinearities by reducing the interaction length between the signal and noise.
One dispersion management technique is known as dispersion mapping, in which the transmission line is divided into two or more sections approximately equal length. In one section, the optical fiber has a zero dispersion wavelength less than the operating wavelengths. The following section has optical fiber with a zero dispersion wavelength greater than the operating wavelengths. The overall transmission line is thus constructed in a periodic manner from a concatenation of fiber sections having different zero dispersion wavelengths. As a result, nonlinear mixing is minimized by reducing the interaction lengths (i. e. the distance over which there is a good group velocity match) and the distortion to the data is minimized as well. By ensuring that the total accumulated dispersion returns to about zero at the receiving end of the system, temporal distortions of the dispersion itself are also minimized.
FIG. 1 shows a dispersion map for a single wavelength system in which adjacent fiber spans have equal dispersion magnitudes but opposite signs. Each span in this example is 50 km in length. As shown, the accumulated dispersion returns to zero at the remote end of the system. One factor complicating the dispersion map shown in FIG. 1 is that optical fiber generally has a nonzero dispersion slope. That is, different wavelengths experience different dispersion values in a given fiber. As a result, in a WDM transmission system employing a plurality of wavelengths, the dispersion map can return the accumulated dispersion to zero for only one wavelength. Thus, in FIG. 1 the operating wavelength is selected to coincide with the end-to-end zero dispersion wavelength for the system.
FIG. 2 shows a dispersion map for three different wavelengths. As a result of the fiber's non-zero dispersion slope, only one wavelength (the center wavelength) has an accumulated dispersion that is periodically returned to zero. The remaining wavelengths accumulate dispersion at different rates, thus causing the spread in accumulated dispersion seen in FIG. 2 as the wavelengths progress along the transmission path.
In WDM transmission systems operating at high data rates the dispersion slope becomes an important factor limiting system capacity. While conventional dispersion maps adequately manage the dispersion, they fail to successfully manage the dispersion slope, leading to an accumulation of dispersion at wavelengths other than the zero dispersion wavelength of the system.